1. Field of the Invention
The present invention relates to composite type membranes which may be used in various membrane processes. The membrane includes a microporous support which is coated with an aqueous polyeurethane dispersion. The present invention also relates to a method for making such membranes and using such membranes in membrane processes.
2. Description of the Prior Art
Coated membranes and membrane processes are used widely in many fields of technology. These processes generally involve the permeation of gases or liquids through polymeric membranes wherein the membrane prevents hydrodynamic flow so that the transport therethrough is controlled by absorption or diffusion.
Membranes are typically selected on how they transport the fluids therethrough. The rate of transport through a membrane is a function of its permeability and is generally referred to as flux. Liquid permeation for example, involves the permeation of feed components from the liquid phase on one side of the membrane to the liquid phase on the other side at a controlled rate.
As discussed, the selectivity of coated membranes is an important factor in the satisfactory operation of the membrane processes, which may include separation or delayed diffusion. In addition, membrane properties such as flux and resistance to chemical, biological and physical degradation also effect the efficiency of such processes.
Of course, there have been many efforts to develop composite membranes which function efficiently for specific processes. Typical of such efforts include the development of composite type membranes such as those disclosed in U.S. Pat. Nos. 4,242,159, 4,260,652, 4,277,344, and 4,388,189. These membranes include a microporous support having coated thereon a thin layer of polymeric material. However, previously known composite membranes have not been completely satisfactory, since they can or may exhibit a variety of defects that affect flux and physical, chemical and biological degradation resistance and thus the overall efficiency of the membrane processes for which they are used.
In an effort to overcome the above described deficiencies in composite type membranes, there have recently been attempts to produce composite type membranes wherein the polymeric coating is a UV curable material. The basic UV curable formulation generally includes a UV reactive unsaturated polymeric material, a photocatalyst and a reactive diluent.
For example, Japanese Kokai Patent No. Sho 59-76504(1984) describes a reverse osmosis membrane which is manufactured by impregnating a porous support with a mixture of monofunctional monomer and bifunctional monomer and irradiating the mixture with light to polymerize the monomers. Furthermore, U.S. Pat. No. 4,618,533 suggests the membrane coated with polymeric material may be cured using ultraviolet light. U.S. Pat. Nos. 4,976,897 and 5,102,552 describe a composite membrane having a microporous support which is coated with a UV curable polymeric composition having a sufficiently high viscosity to prevent pore filling upon coating and curing. The UV curable resin coated composite membrane exhibited suitable resistance to physical, chemical and biological degradation while exhibiting adequate flux for specific uses.
Another approach to application of a coating to the microporous membrane is to change the surface of the hydrophobic microporous membrane to a hydrophilic. This is especially true when polyolefinic films, a preferred type of polymeric material often employed in the manufacture of microporous membranes, are employed. Because these films are not "wetted" with water and aqueous solutions they could not be used advantageously in various applications. Such proposals have been put forth in the past to overcome these problems, such as exemplified by U.S. Pat. Nos. 3,853,601; 3,231,530; 3,215,486 and Canadian Patent No. 981,991 which utilize a variety of hydrophilic coating agents or impregnants. Such coating agents or impregnants, although effective for a limited period of time, tend to be removed in a relatively short period of time by solutions which contact the membrane.
Others have attempted to impart hydrophilic character to a normally hydrophobic microporous membrane by the use of low energy plasma treatments. Such plasma treatments are achieved by first activating surface sites of the microporous membrane using argon or hydrogen plasma and then grafting thereto an appropriate free radical polymerizing species such as acrylic acid. Such plasma treatments result in a film having only a surface which is wettable. The surface of the membrane also becomes plugged when wet, which then inhibits or prevents the free flow of water through the interior of the membrane. The unavoidable plugging of the pores renders the membrane unsuitable for certain applications.
Surface modification treatments such as corona treatment is used with microporous membranes for adhesiveness and permeability. For example U.S. Pat. No. 5,085,775 discloses corona treatment of microporous backing material to improve or increase the adhesion to the microporous polysulfone support. U.S. Pat. No. 5,013,439 discloses corona treatment of microporous polymer film to render the films permeable.
Coating polymeric materials on the porous supports followed by curing has generally been found to result in membranes having low flux. The conventional wisdom is that such coatings tend to wick up and fill the pores of the microporous support, thereby producing a membrane having an insufficient flux.
This failure is unfortunate since such polymeric systems have the potential to be tremendously advantageous in the area of membrane processes, since a wide range of chemical and mechanical properties may be built into the polymeric systems, thereby producing membranes having improved resistance to chemical, physical and biological degradation. Also, the simplicity of these systems compared to conventional systems is potentially appealing, in that they involve solventless processes.
The development of a useful polymeric coated composite membrane for application of membrane processes to impart specific flux control to the membrane would therefore be an extremely important and a desirable development.